The GRB afterglow begins the transition from relativistic to Newtonian phase (the so-called “transrelativistic” stage) when the shock wave expands to a larger radius. The evolutionary phase with Lorentz factor \(0.2 \leqslant \Gamma v/c \leqslant 2\)  can be considered as “transrelativistic”, because by this time, the shock velocity has been significantly decreased and the amount of swept-up circumburst medium becomes large enough, while the afterglow evolution in this stage can be described by neither the relativistic self-similar solution proposed by [1]}, nor the Newtonian Sedov-Taylor solution ([2]}; [3]}). Here, numerical calculations based upon dynamical equations proposed by [4]}, which provide a unified framework from relativistic to deep Newtonian stages of shock-wave evolution, are required, with examples shown in Figure REF . And because in late phases GRB afterglows should be adiabatic, it is safe to assume \(\epsilon = 0\)  in our transrelativistic calculations. Figure REF  shows the temporal evolution of \(\Gamma v/c\)  for adiabatic shocks exhibiting various physical parameters, with the trans-relativistic phase occurring during \(T_{90} + \) dozens to hundreds of days. And considering the transrelativistic phase only begins when the swept-up mass is comparable with the mass of the shock wave itself, it is clearly seen that a larger \(E_{\rm iso}\) , a smaller \(A\) , and a larger \(k\)  can lead to a later transition.
